1. Field of the Invention
The present invention relates in general to distortion compensating circuits and more particularly to third order distortion generating circuits used to compensate for nonlinearities in microwave amplifiers.
2. Description of Related Art
Microwave amplifier transmitters in amplifying a carrier signal produce undesirable third order intermodulation distortion signals as by-products. Predistortion circuits have been developed to generate intermodulation signals which are used to cancel the intermodulation by-products generated within the microwave amplifiers. More specifically, intermodulation signals generated in the predistortion circuit are 180.degree. out-of-phase and of equal amplitude with the intermodulation products of the microwave amplifier so that when the two signals are combined the intermodulation signals will cancel each other. Therefore, the carrier signal will be transmitted with little or no distortion.
Examples of such predistortion circuits are given in U.S. Pat. Nos. 4,157,508, 4,513,250, 3,732,502 and 3,825,843. In Hecken, U.S. Pat. No. 4,157,508, a signal cuber is described which comprises a resistive bridge characterized in that a pair of oppositely-poled diodes are connected in parallel with a resistor in each branch of one pair of opposite branches. In Harmon, U.S. Pat. No. 4,513,250, a signal cuber is disclosed for producing third order harmonics which include a split ring mixer consisting of two transformers and four diodes connected in a bridge type arrangment. In U.S. Pat. No. 3,732,502, the distortion generator comprises two RC coupled transistors which generate higher order signals. Lastly, in U.S. Pat. No. 3,825,843, a squarer and multiplier are used to generate third order intermodulations. None of these predistortion circuits, however, are suitable for microwave frequencies.